He served as the head football coach at the University of Oklahoma for one season in 1966 before his sudden death the following spring.

He succeeded Gomer Jones, a longtime Sooner assistant who had failed to match the success of the legendary Bud Wilkinson, but remained as athletic director.

[6] Entering his second season, Mackenzie died at age 37 of a heart attack in late April.

He collapsed at his Norman home after returning from a brief recruiting trip to north Texas during a break in spring practice.

[7][8][9] He was survived by his wife, Sue Newell, and two children, Katheryn and James, Jr.[10] Recently departed assistant Homer Rice, the first-year head coach at Cincinnati, turned down the chance to take over for Mackenzie.